Extrusion molding and photopolymerization are examples of typical conventional methods for manufacturing an optical disk. Extrusion molding is a method in which, for example, molten polycarbonate resin is extruded under high pressure into a mold containing a disk stamper, and the resin is then cured. Photopolymerization, on the other hand, is a method in which flat plastic plates with good optical properties are prepared, a photocuring resin that can be cured by ultraviolet rays is packed between a plate and a stamper, and the photocuring resin is then cured by ultraviolet irradiation.
A conventional example of photopolymerization has been disclosed in Japanese Laid-Open Patent Application 53-86756. In the method described in this publication, a pattern is transferred from a nickel master (hereinafter, "master" in the specification refers to that which corresponds to a stamper) manufactured by electroforming to polymethyl methacrylate, polycarbonate, or the like using ultraviolet ray-curing resin.
The aforementioned stamper is usually manufactured by coating a glass disk (glass negative) with a photoresist, followed by the exposure of the optical disk pattern using an exposure machine referred to as a laser cutting machine, development, and the thick electroformation of nickel on the pattern thus formed.
Optical disk masters featuring the use of silicon wafers have also been employed recently. In the method disclosed in Japanese Laid-Open Patent Application 61-68746, a silicon oxide is formed on a silicon wafer, a photoresist is applied thereon and exposed, the photoresist is developed, and the silicon oxide is then etched to form a master. In the method disclosed in Japanese Laid-Open Patent Application 4-299937, the silicon wafer is directly etched to form a master. In the method noted in Japanese Laid-Open Patent Applications 4-310624 and 4-311833, a master is manufactured using a silicon wafer so as to obtain an optical disk with better recording density. In the method disclosed in Japanese Laid-Open Patent Application 5-62254, an ultraviolet setting resin is used to transfer a pattern from a master consisting of a silicon wafer to a plastic substrate.
However, the aforementioned methods for manufacturing an optical disk by extrusion molding of polycarbonates have poor mass producibility because the disks are extrusion molded one at a time, and it is difficult to obtain high precision reproducibility.
Improved mass producibility has been attempted using extruded polycarbonate sheets which have been punched or the like and processed into the shape of disks. Polycarbonates have substantial optical anisotropy, however, and disks obtained by processing extrusion molded sheets have variable optical properties depending on the direction in which they are read by a pickup. That is, the optical properties vary along the concentric rings of the disk, precluding its use as an optical disk.
Although the use of acrylic with low optical anisotropy rather than polycarbonates results in good optical properties when disks are obtained by the aforementioned extrusion molding and processing, acrylics are highly moisture-absorbing, and the disks tend to suffer from deformation such as warping. Furthermore, when a reflective film, recording film, or the like is vacuum molded on the disk surface, it is extremely time-consuming to remove gas (particularly moisture) in the disk.